fossui 0.1.2

The Fresh, Minimal Flutter UI kit, 39 components. No app shell required. Inspired by coss.com/ui, Cal.com's design system.

The Fresh, Minimal Flutter UI kit. 39 components.
No app shell required. Inspired by coss.com/ui, Cal.com's design system.

fossui is for developers tired of every Flutter app looking like Material. It drops into any app (a bare WidgetsApp, CupertinoApp, or MaterialApp) and reads its own theme first, not replacing yours. The look comes from coss.com/ui, Cal.com's design system: clean, neutral, superellipse corners. One import, one theme, light and dark out of the box.

Important

Unofficial and independent. Not affiliated with or endorsed by Cal.com, Inc. or coss.com. See NOTICE for attribution.

Why fossui

• A look that isn't Material. A neutral, understated aesthetic with superellipse corners, drawn from the coss/Cal.com design language.
• Framework-agnostic. Built on package:flutter/widgets.dart, with no platform channels and no FossApp wrapper. The widgets work under any app shell.
• Reads your theme, not the other way around. Components resolve context.fossTheme first, so they keep their look inside an existing app instead of inheriting whatever the surrounding shell imposes.
• One theme object. A single FossThemeData holds every semantic token: color, type, radius, spacing, shadow, motion. Reskin the whole app, light and dark, in one call.
• Light on dependencies. One runtime dependency and no bundled icon package. A worst-case app referencing all 39 components adds about 430 KB: roughly 357 KB of Dart code, which tree-shakes down to what you actually use, plus the Geist font (74 KB installed, ~35 KB over the wire). Drop five components and the Dart figure falls to 316 KB. Pass your own icons through plain Widget slots.
• Preview-rich docs. Every component's API doc renders a live light and dark preview, not just text, and the same preview shows on hover in your IDE. Each one states plainly what it does and does not do.
• Accessible by default. Semantics, focus, and touch targets are built into each component.

Install

dependencies:
  fossui: ^0.1.2

Or from the command line:

flutter pub add fossui

Browse the package on pub.dev/packages/fossui.

Quick start

Register the theme once, then use the widgets anywhere.

import 'package:flutter/widgets.dart';
import 'package:fossui/fossui.dart';

void main() => runApp(
      FossTheme(
        data: FossThemeData.light,
        child: WidgetsApp(
          color: FossThemeData.light.colors.background,
          builder: (context, _) => Center(
            child: FossButton(
              onPressed: () {},
              child: const Text('Get started'),
            ),
          ),
        ),
      ),
    );

No shell lock-in. FossTheme is a plain InheritedWidget, so context.fossTheme resolves the same way under WidgetsApp, CupertinoApp, or any other app widget. If your app already registers a theme, toThemeData() hands the same tokens over as a theme extension.

See example/ for a runnable app.

Theming

The defaults give fossui its look, but nothing is locked. Read tokens through one accessor:

final theme = context.fossTheme;
final color = theme.colors.primary;
final radius = theme.radii.md;

To reskin the app, layer a FossThemeSpec over a base theme. Every field is optional, and anything you leave unset keeps the default.

FossTheme(
  data: FossThemeData.light.retheme(
    const FossThemeSpec(primary: Color(0xFF16A34A), radius: 22),
  ),
  child: const MyApp(),
);

Components

The library covers input, feedback, overlays, and layout:

Group	Components
Actions and input	Button, TextField, NumberField, OtpField, Select, MultiSelect, Combobox, MultiCombobox, Autocomplete, Checkbox, Radio, Switch, Toggle, ToggleGroup, Chip, Slider, DatePicker, TimePicker, Pagination
Feedback	Alert, Badge, Meter, Progress, Skeleton, Spinner, Toast, Tooltip
Overlays	Dialog, AlertDialog, Drawer (sheet and bottom sheet), Popover
Layout and media	Accordion, Card, ListTile, Tabs, Separator, Text, Calendar, Avatar

See the components roadmap for what is shipped and what is planned, and the component checklist for the bar each one clears.

Icons

Icon slots accept a plain Widget, so any icon set works: Lucide, Cupertino, SVGs, or your own. The package pulls in no icon dependency of its own. The example and the docs use Lucide as the documented companion.

Platforms

With no platform channels, fossui runs anywhere Flutter does: Android, iOS, web, macOS, Windows, and Linux are all supported.

AI-native

fossui runs an MCP server so your AI coding assistant writes fossui code against the real component API instead of guessing prop names and enum values. It works with any MCP client (Claude Code, Cursor, VS Code, and more) over one endpoint:

https://mcp.fossui.org

Per-client setup and the full tool list are in the AI-native docs.

Blog

• Introducing fossui: a minimal Flutter UI kit
• Migrating from Material to fossui
• Minimal Flutter UI kits compared: fossui vs Material (2026)

Ecosystem

• Documentation: fossui.org
• Blog: fossui.org/blogs
• Live gallery: play.fossui.org
• AI-native (MCP): fossui.org/docs/ai-native
• Package: pub.dev/packages/fossui

Development

This package pins its Flutter SDK with fvm:

fvm install          # uses .fvmrc (Flutter 3.41.9)
fvm flutter pub get
fvm flutter test

The test suite runs on every change and holds full line coverage; browse the live report at coverage.fossui.org.

Contributions are welcome. Start with CONTRIBUTING.md and the Code of Conduct.

Acknowledgements

The look and component API are drawn from coss.com/ui, the Cal.com design system, with ideas from Base UI and shadcn/ui. Full attribution is in NOTICE.

Support

If fossui is useful to you, two clicks go a long way:

• ⭐ Star fossui on GitHub
• 👍 Like fossui on pub.dev
• ✉️ Questions or feedback: [e-mail]

Star History